Process for the multi-layered coating of substrates with electrophoretic coating material and powder coating material

ABSTRACT

The present invention relates to a process for the multilayer coating of substrates with electrodeposition and powder coating materials, in which electrodeposition is used to apply at least one coat (2) of electrodeposition coating material to the substrate (1), after deposition the substrate (1) is, if desired, wholly or partially air-dried, a coat of powder coating material (3) is then applied, and finally electrodeposition coating material and powder coating material are jointly baked.

The present invention relates to a process for the multilayer coating ofsubstrates with a primer coat of electrodeposition coating material andwith a topcoat of powder coating material.

The coating of first and foremost electrically conductive substrateswith an electrodeposition coating material is a process which has beencommon for many years. The electrodeposition coating material in thisprocess is present as an (aqueous) dispersion in a bath. The substrateto be coated is connected as one of two electrodes and is lowered intothis bath. This is followed by the electrophoretic deposition of theelectrodeposition coating material on the substrate. After asufficiently thick coat of material has been obtained, the coatingoperation is ended and the coat of material is dried and, generally,baked.

Resins which can be electrodeposited at the cathode are described, forexample, in U.S. Pat. No. 3,617,458. They comprise crosslinkable coatingcompositions which deposit themselves at the cathode. These coatingcompositions are derived from an unsaturated addition polymer whichcomprises amine groups and carboxyl groups and from an epoxidizedmaterial.

U.S. Pat. No. 3,663,389 describes cationically electrodepositablecompositions which are mixtures of specific amine-aldehyde condensatesand a large number of cationic resinous materials, one of thesematerials being preparable by reacting an organic polyepoxide with asecondary amine and solubilizing the product with acid.

U.S. Pat. No. 3,640,926 discloses aqueous dispersions which can beelectrodeposited at the cathode and consist of an epoxy resin ester,water and tertiary amino salts. The epoxy ester is the reaction productof a glycidyl polyether and a basic unsaturated oleic acid. The aminesalt is the reaction product of an aliphatic carboxylic acid and atertiary amine.

Epoxy- and polyurethane-based binders for use in binder dispersions andpigment pastes are, moreover, known in numerous configurations.Reference may be made, for example, to DE-27 01 002, EP-A-261 385,EP-A-004 090 and DE-C 36 30 667.

The coating of substances with powder coating materials is also a commonprocess. In this case, the dry, pulverulent coating material is applieduniformly to the substrate that is to be coated. Subsequently, throughheating of the substrate, the coating material is melted and baked. Theparticular advantages of powder coating materials are, inter alia, thatthey manage without solvents and that the overspray losses which occurwith conventional coating materials are avoided, since virtually all ofthe nonadhering powder coating material can be recycled. The powdercoating is applied to the substrate preferably by electrostaticadhesion, generated through the application of high voltage or byfrictional charging.

Combination coating with electrodeposition coating material and powdercoating material is also known from the prior art. For example, inaccordance with DE-C 4313762, a powder coat is first of all sintered onand then an electrodeposition coating material is applied. It is alsoknown, from JP 63274800, to apply an electrodeposition coating materialand to dry it at 110° C., to apply a powder coating material, and,finally, to jointly bake both coats. This two-coat or multicoat systemenables the product properties to be optimized. Priming withelectrodeposition coating material may also become necessary in the caseof substrates which, for technical reasons related to their material oron geometric grounds, are relatively unaminable to powder coatingmaterial. A typical application of this multicoat system is the coatingof heating-system radiators. The procedure here is such that, followingthe coating of the substrate with the electrodeposition coatingmaterial, said coating material is first baked in a drier. Thetemperatures in the drier typically reach more than 100° C., and theelectrodeposition coating material sets. Following this bakingoperation, the primed substrate is cooled again before then beingprovided with the powder coat. A second baking operation is thennecessary to cure the applied powder coating material. The disadvantageof this procedure is that the substrate has to be twice dried and heatedduring the coating operation. This is very energy-intensive, and entailsconsiderable capital and operating costs.

Against the background of this prior art, the invention has set itselfthe object of developing a process for the multilayer coating ofsubstrates with electrodeposition and powder coating materials whichoperates more simply, more cost-effectively and with greater energysavings while maintaining identical product qualities. This object isachieved in accordance with the invention by a process in which

a) to a substrate (1) made preferably of metal, especially iron or zinc,at least one coat (2) of liquid coating material, preferablyelectrodeposition coating material, is applied,

b) after deposition the substrate (1) is, if desired, wholly orpartially dried,

c) at least one coat of powder coating material (3) is applied, and

d) electrodeposition coating material and powder coating material arejointly baked,

where drying takes place at temperatures of ≦100° C., preferably ≦40° C.

The process of the invention therefore omits a separate drying andbaking step for the electrodeposition coating material before the powdercoating material is applied. Instead, both coating materials are bakedin a joint step. This approach represents a considerable simplificationof the coating operation. The omission of one baking operation reducesboth the capital costs and the operating costs. Only a single bakingoven needs to be provided and operated. As a result, there is also asaving of heating energy. In addition, the overall processing time forthe coating operation is shorter, and so the productivity of the unit isincreased.

Since the substrate to be coated is preferably preprimed with anelectrodeposition coat, said substrate is principally an electricallyconductive substrate. In particular, it can be a metal, preferably ironor zinc.

In step a), in accordance with the invention, a liquid coating materialis applied to the above-described substrate. This can be done using allcoating techniques known in the prior art.

As the coating material it is possible to use all liquid coatingmaterials which are known in the art. Suitable in particular are allcustomary aqueous electrodeposition coating materials. It is possible,for example, to use electrodeposition coating materials which compriseepoxy resins, which are preferably amine-modified, and/or blockedaliphatic polyisocyanate, pigment paste and, if desired, furtheradditives.

In a preferred embodiment of the process of the invention theelectrodeposition coat, following removal of the substrate from thebath, is predried, preferably by air drying with the aid, for example,of a fan. The air may preferably be dry air, e.g. compressed air.

Simultaneously with the drying operation, gentle heating of thesubstrate is performed in the course of which, however, flow or bakingof the coating material must be avoided. The primary aim, rather,is—when using the customary aqueous electrodeposition coatingmaterials—to remove the film of water remaining thereon. For thisreason, temperatures of ≦100° C. are preferred. Preferably, temperaturesof ≦80° C., with particular preference ≦60° C. and, most preferably, of≦40° C. should be observed.

The drying operation extends over a period of not more than 60 minutes.The drying time is preferably ≦40 minutes, with particular preference≦30 minutes and, most preferably, ≦20 minutes.

The predrying of the electrodeposition coat is preferably performeduntil its content of solvents has fallen such that on subsequent bakingthe substance of the coat decreases by less than 20%, preferably lessthan 13%, this is because, when baking an electrodeposition coat, thereis always a loss of substance through the evaporation of residualsolvents and through the emission of elimination products which formduring the crosslinking of the coating material. The gaseous expulsionof these substances may result in bubbles being formed, so that the coatof material overall is destroyed. If predrying is carried out up to themaximum limits of the solvent content as indicated above, however, thegaseous expulsion of the residual solvents and of the eliminationproducts does not lead to any deterioration in product quality.

In accordance with the prior art the baking of the electrodepositioncoat has been carried out before application of the powder coatingmaterial, in order to avoid the above-described degassing phenomena. Inthe view of those skilled in the art, it was not considered possible toapply the powder coating material to an unbaked electrodeposition coatwithout both coats being destroyed by the degassing process. Thisprejudice has been overcome with the process of the invention.

A powder coating material is applied, in accordance with the invention,to the abovementioned electrodeposition coating material.

The essential factor is that the crosslinking temperatures of the powdercoating material are higher than those of the electrodeposition coatingmaterial. Preferably, the temperature difference is from 5 to 60° C.,with particular preference from 10 to 40° C., with very particularpreference from 10 to 30° C. and, most preferably, from 10 to 20° C.

All known coating formulations are suitable in accordance with theinvention: for example those described in EP-509 392, EP-509 393, EP-322827, EP-517 536, U.S. Pat. Nos. 5,055,524 and 4,849,283. In particular,the powder coating material can consist of epoxy resins, also epoxidizedNovolaks, of crosslinking agents, preferably phenolic or amine-typehardeners or bicyclic guanidines, catalysts, fillers and, if desired,auxiliaries and additives.

The powder coating materials employed in accordance with the inventionpreferably comprise epoxy resins, phenolic crosslinking agents,catalysts, assistants and also, if desired, auxiliaries andpowder-typical additives, and flow aids.

Suitable epoxy resins are all solid epoxy resins having an epoxyequivalent weight of between 400 and 3000, preferably from 600 to 2000.These are principally epoxy resins based on bisphenol A and bisphenol F.Preference is given to epoxidized Novolak resins. These preferably havean epoxide equivalent weight of from 500 to 1000.

The epoxy-resins based on bisphenol A and bisphenol F generally have afunctionality of less than 2, the epoxidized Novolak resins afunctionality of more than 2. Particular preference is given in thepowder coating materials of the invention to epoxidized Novolak resinshaving an average functionality in the range from 2.4 to 2.8 and havingan epoxide equivalent weight in the range from 600 to 850. In the caseof the epoxidized Novolak resins, the phenolic hydroxyl groups areetherified with alkyl, acrylic or similar groups. By reacting thephenolic hydroxyl groups with epichlorohydrides [sic], epoxide groupsare introduced into the molecule. This procedure, starting fromNovolaks, forms the so-called epoxy-Novolak. The epoxidized Novolaks arestructurally related to bisphenol A resins. Epoxidized Novolak resinscan be prepared by epoxidizing Novolaks which consist, for example, offrom 3 to 4 phenol nuclei connected to one another by way of methylenebridges. Alkyl-substituted phenols which are reacted with formaldehydecan also be used as Novolak resins.

Examples of suitable epoxy resins are the products obtainablecommercially under the following names:

Epikote 1004, 1055, 3003, 3004, 2017 from Shell-Chemie, DER 640, 671,662, 663U, 664, 667 from Dow, and Araldit GT 6063, 6064, 6084, 6097,7004, 7220, 7225 from Ciba Geigy.

Examples of a suitable epoxy-functional binder for the transparentpowder coating materials are epoxy-functional polyacrylate resins whichcan be prepared by copolymerizing at least one ethylenically unsaturatedmonomer which comprises at least one epoxide group in the molecule withat least one further ethylenically unsaturated monomer which contains noepoxide group in the molecule, at least one of the monomers being anester of acrylic acid or methacrylic acid.

Epoxy-functional polyacrylate resins are known (cf. e.g. EP-A-299 420,DE-B-22 14 650, DE-B-27 49 576, U.S. Pat. Nos. 4,091,048 and 3,781,379).

Examples of the ethylenically unsaturated monomers which comprise atleast one epoxide group in the molecule are glycidyl acrylate, glycidylmethacrylate and allyl glycidyl ether.

Examples of ethylenically unsaturated monomers which contain no epoxidegroup in the molecule are alkyl esters of acrylic and methacrylic acidwhich contain 1 to 20 carbon atoms in the alkyl radical, especiallymethyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methylacrylate 2-ethylhexyl acrylateand 2-ethylhexyl methacrylate. Further examples of ethylenicallyunsaturated monomers which contain no expoxide groups in the moleculeare acids, such as acrylic acid and methacrylic acid, acid amides, suchas acrylamide and methacrylamide, vinylaromatic compounds, such asstyrene, methylstyrene and vinyltoluene, nitriles, such as acrylonitrileand methacrylonitrile, vinyl halides and vinylidene halides, such asvinyl chloride and vinylidene fluoride, vinyl esters, such as vinylacetate, and hydroxyl-containing monomers, such as hydroxyethyl acrylateand hydroxyethyl methacrylate, for example.

The epoxy-functional polyacrylate resin normally has an epoxideequivalent weight of from 400 to 2500, preferably from 500 to 1500 and,with particular preference, from 600 to 1200, a number-average molecularweight (determined by gel permeation chromatography using a polystyrenestandard) of from 1000 to 15,000, preferably from 1200 to 7000 and, withparticular preference, from 1500 to 5000, and a glass transitiontemperature (T_(g)) of from 30 to 80, preferably from 40 to 70 and, withparticular preference, from 50 to 70° C. (measured with the aid ofdifferential scanning calorimetery (DSC)).

The epoxy-functional polyacrylate resin can be prepared by generallywell-known methods, by free-radical addition polymerization.

Examples of suitable hardeners for the epoxy-functional polyacrylateresin are polyanhydrides of polycarboxylic acids or of mixtures ofpolycarboxylic acids, especially polyanhydrides of dicarboxylic acids orof mixtures of dicarboxylic acids.

Polyanhydrides of this kind can be prepared by removing water from thepolycarboxylic acid or mixture of polycarboxylic acids, with twocarboxyl groups being reacted in each case to form one anhydride group.Preparation techniques of this kind are well known and thus require nofurther elucidation.

For the curing of the epoxy resins, the powder coating material of theinvention comprises phenolic or amine-type hardeners. Bicyclicguanidines may also be employed.

In this context it is possible, for example, to use any desired phenolicresin provided it has the methylol functionality required forreactivity. Preferred phenolic resins are products, prepared underalkaline conditions, of the reaction of phenol, substituted phenols andbisphenol A with formaldehyde. Under such conditions the methylol groupis linked to the aromatic ring in either ortho or para position. Inaccordance with the present invention, the phenolic crosslinking agentsemployed are, with particular preference, hydroxyl-containing bisphenolA resins or bisphenol F resins having a hydroxy equivalent weight in therange from 180 to 600 and, with particular preference, in the range from180 to 300. Phenolic crosslinking agents of this kind are prepared byreacting bisphenol A or Bisphenol F with glycidyl-containing components,such as, for example, with the diglycidyl ether of bisphenol A. Phenoliccrosslinking agents of this kind are obtainable, for example, under thecommercial designation DEH 81, DEH 82 and DEH 87 from Dow, DX 171 fromShell-Chemie and XB 3082 from Ciba Geigy.

In this context, the epoxy resins and the phenolic crosslinking agentsare employed in such a ratio that the number of epoxide groups to thenumber of phenolic OH groups is approximately 1:1.

The powder coating materials of the invention comprise one or moresuitable catalysts for epoxy resin curing. Suitable catalysts arephosphonium salts of organic or inorganic acids, imidazole and imidazolederivatives, quaternary ammonium compounds, and amines. The catalystsare generally employed in proportions of from 0.001% by weight to about10% by weight, based on the overall weight of the epoxy resin and of thephenolic crosslinking agents.

Examples of suitable phosphonium salt catalysts areethyltriphenylphosphonium iodide, ethyltriphenylphosphonium chloride,ethyltriphenylphosphonium thiocyanate, ethyltriphenylphosphoniumacetate-acetic acid complex, tetrabutylphosphonium iodide,tetrabutylphosphonium bromide and tetrabutylphosphonium acetateaceticacid complex. These and other suitable phosphonium catalysts aredescribed, for example, in U.S. Pat. Nos. 3,477,990 and 3,341,580.

Examples of suitable imidazole catalysts are 2-styrylimidazole,1-benzyl-2-methylimidazole, 2-methylimidazole and 2-butylimidazole.These and other imidazole catalysts are described, for example, inBelgian Patent No. 756,693.

In some cases, customary commercial phenolic crosslinking agents alreadyinclude catalysts for epoxy resin crosslinking.

Powder coating materials based on carboxyl-containing polyesters and onlow molecular mass crosslinking agents containing epoxide groups areknown in large numbers and are described, for example, in EP-A-389 926,EP-A-371 522, EP-A-326 230, EP-B-110 450, EP-A-110 451, EP-B-107 888,U.S. Pat. No. 4,340,698, EP-B-119 164, WO 87/02043 and EP-B-10 805.

Particularly suitable are powder coating materials according to DE 43 30404.4, which comprise as film-forming material

A) 35.0-92.2% by weight of carboxyl-containing polyesters having an acidnumber of 10-150 mg of KOH/g,

B) 0.8-20.1% by weight of low molecular mass curing agents containingepoxide groups,

C) 3.7-49.3% by weight of epoxy-functional polyacrylate resins having anepoxide equivalent weight of 350-2000, and

D) 0.5-13.6% by weight of low molecular mass di- and/or polycarboxylicacids and/or di- and/or polyanhydrides,

the sum of the proportions by weight of A), B), C) and D) being in eachcase 100% by weight and the ratio of the epoxide groups of the powdercoating materials to the sum of the carboxyl and anhydride groups of thepowder coating materials being 0.75-1.25:1.

The carboxyl-containing polyesters used as component A) have an acidnumber in the range of 10-150 mg of KOH/g, preferably in the range of30-100 mg of KOH/g. The hydroxyl number of the polyester resins shouldbe ≦30 mg of KOH/g. Preference is given to employing polyesters having acarboxy functionality of ≧2. The polyesters are prepared by thecustomary methods (compare e.g. Houben Weyl, Methoden der OrganischenChemie, 4th Edition, Volume 14/2, Georg Thieme Verlag, Stuttgart 1961).

Suitable as a carboxylic acid component for preparing the polyesters arealiphatic, cycloaliphatic and aromatic di- and polycarboxylic acids,such as phthalic acid, terephthalic acid, isophthalic acid, trimelliticacid, pyromellitic acid, adipic acid, succinic acid, glutaric acid,pimelic acid, suberic acid, cyclohexanedicarboxylic acid, azelaic acid,sebacic acid and the like. These acids can also be employed in the formof their esterifiable derivatives (e.g. anhydrides) or of theirtransesterifiable derivatives (e.g. dimethyl esters).

As an alcohol component for preparing the carboxyl-containing polyestersA), the commonly employed di- and/or polyols are suitable, examplesbeing ethylene glycol, propane-1,2-diol and propane-1,3-diol, butanediols, diethylene glycol, triethylene glycol, tetraethylene glycol,hexane-1,6-diol, neopentyl glycol, 1,4-dimethylolcyclohexane, glycerol,trimethylolethane, trimethylolpropane, pentaerythritol,ditrimethylolpropane, dipentaerythritol, diglycerol and the like.

The polyesters thus obtained can be employed individually or as amixture of different polyesters. The polyesters suitable as component A)generally have a glass transition temperature of more than 30° C.

Examples of suitable commercial polyesters are the products obtainablecommercially under the following trade names: Crylcoat 314, 340, 344,2680, 316, 2625, 320, 342 and 2532 from UCB, Drogenbos, Belgium;Grilesta 7205, 7215, 72-06, 72-08, 72-13, 72-14, 73-72, 73-93 and 7401from Ems-Chemie; Neocrest P670, P671, P672, P678, P662 from ICI, andUralac P2400, P2450, P5980, PS 998, P 3561 Uralac P3400 and Uralac P5000from DSM.

Also suitable as an acidic polyester component A) are unsaturated,carboxyl-containing polyester resins. These are obtained bypolycondensation of, for example, maleic acid, fumaric acid or otheraliphatic or cycloaliphatic dicarboxylic acids having an ethylenicallyunsaturated double bond, together if desired with saturatedpolycarboxylic acids, as polycarboxylic acid component. The unsaturatedgroups can also be introduced into the polyester through the alcoholcomponent, e.g. by trimethylolpropane monoallyl ether.

The powder coating materials of the invention comprise as component B)0.8-20.1% by weight of low molecular mass curing agents containingepoxide groups. An example of a particularly suitable low molecular masscuring agent containing epoxide groups is triglycidyl isocyanurate(TGIC). TGIC is obtainable commercially, for example, under thedesignation Araldit PT 810 (manufacturer: Ciba Geigy). Further suitablelow molecular mass curing agents containing epoxide groups are1,2,4-triglycidyltriazoline-3,5-dione, diglycidyl phthalate, and thediglycidyl ester of hexahydrophthalic acid.

By epoxy-functional polyacrylate resins (component C) are meant polymerswhich can be prepared by copolymerizing at least one ethylenicallyunsaturated monomer which comprises at least one epoxide group in themolecule with at least one further ethylenically unsaturated monomerwhich contains no epoxide group, at least one of the monomers being anester of acrylic acid or methacrylic acid.

Epoxy-functional polyacrylate resins are known (cf. e.g. EP-A-299 420,DE-B-22 14 650, U.S. Pat. Nos. 4,091,048 and 3,781,379).

Examples of the ethylenically unsaturated monomers which comprise atleast one epoxide group in the molecule are glycidyl acrylate, glycidylmethacrylate and allyl glycidyl ether.

Examples of ethylenically unsaturated monomers which contain no epoxidegroup in the molecule are alkyl esters of acrylic and methacrylic acidwhich contain 1 to 20 carbon atoms in the alkyl radical, especiallymethyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate andthe corresponding methacrylates, 2-ethylhexyl acrylate and 2-ethylhexylmethacrylate. Further examples of ethylenically unsaturated monomerswhich contain no expoxide groups in the molecule are acids, such asacrylic acid and methacrylic acid, acid amides, such as acrylamide andmethacrylamide, vinylaromatic compounds, such as styrene, methylstyreneand vinyltoluene, nitriles, such as acrylonitrile and methacrylonitrile,vinyl halides and vinylidene halides, such as vinyl chloride andvinylidene fluoride, vinyl esters, such as vinyl acetate and vinylpropionate, and hydroxyl-containing monomers, such as hydroxyethylacrylate and hydroxyethyl methacrylate, for example.

The epoxy-functional polyacrylate resin (component C) has an epoxideequivalent weight of from 350 to 2000. Usually, the epoxy-functionalpolyacrylate resins have a number-average molecular weight (determinedby gel permeation chromatography using a polystyrene standard) of from1000 to 15,000, and a glass transition temperature (T_(gn)) of 30-80(measured with the aid of differential scanning calorimetry (DSC)).

The epoxy-functional acrylate resin can be prepared by generallywell-known methods, by free-radical addition polymerization.Epoxy-functional polyacrylate resins of this kind are obtainablecommercially, for example, under the designation Almatex PD 7610 andAlmatex PD 7690 (manufacturer: Mitsui Toatsu).

As binders, the powder coating materials of the invention comprise ascomponent D) 0.5-13.6% by weight of low molecular mass di- and/orpolycarboxylic acids and/or di- and/or polyanhydrides. It is preferredas component D) to use saturated, aliphatic and/or cycloaliphaticdicarboxylic acids, such as glutaric acid, adipic acid, pimelic acid,suberic acid, azelaic acid, cyclohexanedicarboxylic acid, sebacic acid,malonic acid, dodecanedioic acid and succinic acid. Also suitable,furthermore, as component D) are aromatic di- and polycarboxylic acids,such as phthalic acid, terephthalic acid, isophthalic acid, trimelliticacid and pyromellitic acid, also of course in the form of theiranhydrides where they exist. Particular preference is given to using ascomponent D) dodecandioic acid (=1,10-decanedicarboxylic acid).

The amounts of the powder coating components A) to D) are chosen suchthat the ratio of the epoxide groups from B) and C) to the sum of thecarboxyl and anhydride groups from A) and D) is 0.75-1.25:1. This ratiois preferably 0.9-1.1:1.

The powder coating materials comprise from 50 to 90%, preferably from 60to 80% by weight of binder and from 10 to 50% by weight, preferably from20 to 40% by weight of fillers.

Suitable fillers are glycidyl-functionalized, crystalline silicamodifications. They are normally employed in the stated range of from 10to 50% by weight, based on the overall weight of the powder coatingmaterial. In some cases, however, filler contents of more than 50% byweight are also possible.

The crystalline silica modifications include quartz, cristobalite,tridymite, keatite, stishovite, melanophlogite, coesite and fibroussilica. The crystalline silica modifications areglycidyl-functionalized, the glycidyl functionalization being obtainedby surface treatment. The silica modifications concerned are, forexample, based on quartz, cristobalite and fuzed silica and are preparedby treating the crystalline silica modifications with epoxy silanes. Theglycidyl-functionalized silica modifications are obtainable on themarket, for example, under the designation Silbond^(R) 600 EST andSilbond^(R) 6000 EST (manufacturer: Quarzwerke GmbH) and are prepared byreacting crystalline silica modifications with epoxy silanes.

The powder coating materials advantageously comprise from 10 to 40% byweight, based on the overall weight of the powder coating material, ofglycidyl-functionalized crystalline silica modifications.

The powder coating materials may also comprise further inorganicfillers, examples being titanium oxide, barium sulfate andsilicate-based fillers, such as talc, kaolin, magnesium silicates,aluminum silicates, micas and the like. The powder coating materialsmay, furthermore, if desired, contain auxiliaries and additives as well.Examples of these are leveling agents, flow aids and degassing agents,such as benzoin, for example.

To assist nondestructive gas expulsion, finally, degassing agents can beadded to the powder coating material. The concentrations of thisdegassing agent are preferably ≦2% by weight, with particular preferencefrom 0.1 to 0.8% by weight, with very particular preference from 0.2 to0.5% by weight, and most preferably, ≦0.4% by weight.

Particularly suitable degassing agents are compounds of the formula

in which R is an alkanol having 1-6 carbon atoms. In this formula, R₁and R₂ are benzoyl—or phenyl groups. R₁ and R₂ may, moreover, beidentical or different. In other words, R₁ and R₂ can both be benzoyl orphenyl groups, respectively. Likewise, one radical can be a benzoylgroup while the other radical is a phenyl group. Examples of compoundswhich can be employed with preference is benzoylphenylmethanol(benzoin).

The powder coating materials are prepared by known methods (cf. e.g.Product information from BASF Lacke+Farben AG, “Pulverlacke” [Powdercoating materials], 1990) by homogenization and dispersion by means, forexample, of an extruder, screw compounder and the like. Followingpreparation of the powder coating materials, they are adjusted to thedesired particle size distribution by milling, and if appropriate, bysieving and classifying.

The powder coating materials described are, following application, bakedjointly with the electrodeposition coat. Baking of the electrodepositionand powder coats is accompanied by melting of the powder coatingmaterial and, consequently, by its equal distribution, and by curing ofthe binders. Baking is preferably conducted at temperatures of from 150to 220° C. and, with very particular preference, at from 160 to 200° C.This baking operation last for from 10 to 40 minutes, preferably from 15to 30 minutes.

Methods suitable for applying the powder coating material are all commonprior art methods. Particular preference is given to application byelectrostatic adhesion, preferably by applying a high voltage or byfrictional charging.

The process of the invention finds a preferred application in connectionwith the coating of radiators, car bodies and automotive accessories,machine components, compressors, shelving units, office furniture andcomparable industrial products.

The invention also provides a multilayer-coated substrate which isprepared by first applying a coat of electrodeposition coating materialto the substrate in an electrodeposition coating bath and then, ifdesired, drying it, subsequently applying a coat of powder coatingmaterial and, finally, jointly baking electrodeposition coating materialand powder coating material in one step.

The electrodeposition coat of the multiply coated substrate of theinvention preferably has a thickness of from 5 to 35 μm, with veryparticular preference from 10 to 25 μm. The powder coat preferably has athickness of from 30 to 200 μm, with very particular preference from 50to 120 μm.

The implementation of the process of the invention and the preparationof the substrate of the invention are shown diagrammatically in FIGS. 1and 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the layer structure of the substrate.

FIG. 2 shows the preparation steps.

FIG. 1 shows diagrammatically the layer structure of the substrate ofthe invention. On the substrate 1 itself there is located, first of all,the coat 2 of electrodeposition coating material, which is covered by ausually 10 times thicker coat 3 of powder coating material. For thepreparation of the substrate of the invention, the substrate is first ofall coated in an electrodeposition coating bath 4. It is then removedfrom the electrodeposition coating bath and dried in a drying unit 5 byblowing with air. Subsequently, and with, for example, application of ahigh voltage in a booth 6, powder coating material is sprayed in finelydivided form onto the surface of the substrate. This powder coatingmaterial is then baked jointly in the oven 7 with the electrodepositioncoat at temperatures of from about 150 to 220° C.

In the text below the process of the invention is elucidated furtherwith reference to an example.

1. Preparing an Amine-modified Epoxy Resin which Has Active HydrogenAtoms

A reaction vessel is charged with 1780 g of Epikote 1001 (epoxy resinfrom Shell having an epoxide equivalent weight of 500), 280 g ofdodecylphenol and 105 g of xylene and this initial charge is melted at120° C. under a nitrogen atmosphere. Subsequently, under a gentlevacuum, traces of water are removed through an extraction circuit. Then3 g of N,N-dimethylbenzylamine are added, the reaction mixture is heatedto 180° C. and this temperature is maintained for about 3 h until theepoxide equivalent weight (EEW) has risen to 1162. The mixture is thencooled, and 131 g of hexyl glycol, 131 g of diethanolamine and 241 g ofxylene are added in rapid succession. During these additions, thetemperature rises slightly. Subsequently, the reaction mixture is cooledto 90° C. and diluted further with 183 g of butyl glycol and 293 g ofisobutanol. When the temperature has fallen to 70° C., 41 g ofN,N-dimethylaminopropylamine are added, this temperature is maintainedfor 3 h, and the product is discharged.

The resin has a solids content of 70.2% and a base content of 0.97milliequivalent/gram.

2. Preparing a Blocked Aliphatic Polyisocyanate

A reaction vessel is charged under a nitrogen atmosphere with 488 g ofhexamethylene diisocyanate which has been trimerized by isocyanurateformation (commercial product of BASF AG, having an isocyanateequivalent weight of 193) and with 170 g of methyl isobutyl ketone, andthis initial charge is heated to 50° C. Then 312 g of di-n-butylamineare added dropwise at a rate such that the internal temperature is heldat from 60 to 70° C. Following the end of the addition, stirring iscontinued at 75° C. for 1 h and then the reaction mixture is dilutedwith 30 g of n-butanol and cooled. The reaction product has a solidscontent of 79.6% (1 h at 130° C.) and an amine number of less than 5 mgof KOH/g.

3. Preparing an Aqueous Dispersion which Comprises a Cationic,Amine-modified Epoxy Resin Containing Active Hydrogen Atoms and aBlocked Aliphatic Polyisocyanate as Separate Component

1120 g of the resin solution prepared in section 1. are mixed at roomtemperature and with stirring with 420 g of the solution of the blockedpolyisocyanate prepared in section 2. As soon as the mixture ishomogeneous (after about 15 minutes), 2.2 g of a 50% strength by weightsolution of a customary commercial antifoam (Surfynol; commercialproduct of Air Chemicals) in ethylene glycol monobutyl ether and 18 g ofglacial acetic acid are stirred in. Subsequently, 678 g of deionizedwater, divided into 4 portions, are added. Subsequently, dilution iscarried out with a further 1154 g of deionized water in small portions.

The resulting aqueous dispersion is freed from low-boiling solvents byvacuum distillation and then diluted with deionized water to a solidscontent of 33% by weight.

4. Preparing a Grinding Resin in Accordance with DE-A-34 22 457

640 parts of a diglycidyl ether based on bisphenol A and epichlorohydrinand having an epoxide equivalent weight of 485 and 160 parts of asimilar compound having an epoxide equivalent weight of 189 are mixed at100° C. A further vessel is charged with 452 parts ofhexamethylenediamine, this initial charge is heated to 100°0 C., and 720parts of the above hot epoxy resin mixture are added over the course ofone hour, during which it is necessary to carry out gentle cooling inorder to maintain the temperature at 100° C. After a further 30 minutesthe excess hexamethylenediamine is stripped off under reduced pressureand elevated temperature, toward the end the temperature reaching 205°C. and the pressure 30 mbar. Subsequently, 57.6 parts of stearic acid,172.7 parts of dimeric fatty acid and 115 parts of xylene are added.Then the water formed is distilled off azeotropically over 90 minutes atfrom 175 to 180° C. Subsequently, 58 parts of butyl glycol and 322 partsof isobutanol are added. The product has a solids content of 70% byweight and a viscosity, measured at 75° C. with a cone-and-plateviscometer, of 2240 mPas.

5. Preparing a Pigment Paste

586 parts of the grinding resin prepared in section 4. are mixedthoroughly with 990 parts of deionized water and 22 parts of glacialacetic acid. This mixture is subsequently combined with 1129 parts ofTiO₂ and 146 parts of an extender based on aluminum silicate. Thismixture is comminuted in a milling apparatus to a Hegman fineness ofless than 12 μm. Subsequently, deionized water is added until a solidscontent of from 48 to 52% by weight (1/2 h, 180° C.) has been reached.

6. Preparing an Electrodeposition Coating Bath which is Employed inAccordance with the Invention

810 parts by weight of the pigment paste prepared in section 5. areadded to 2200 parts by weight of the dispersion prepared in section 3.,and the mixture is made up to 5000 parts by weight with deionized water.

7. Preparing a Powder Coating Material Employed in Accordance with theInvention (More on Page 31a)

8. Coating Process According to the Invention

A flat radiator of height 600 mm and length 1000 mm, comprising 2 panelsonto which 1 convector plate in each case is internally welded, isdegreased and phosphatized and then lowered into an electro-depositioncoating bath and connected as the cathode.

Parameters

Voltage between 100 and 400 V, preferably from 150 to 300 V

Temperature from 24 to 35° C., preferably from 28 to 32° C.

Time from 120 to 300 s, preferably from 150 to 240 s.

The radiator is then rinsed and blown with air until no further liquiddrips off. The radiator is then externally coated with powder and bakedin a drying oven from 150 to 220° C., preferably at from 160 to 200° C.,for from 10 to 40 minutes, preferably from 15 to 30 minutes.

In order for the resulting powder coating film to exhibit no defects, aslittle as possible of elimination products and solvents should escapefrom the CED material during this baking operation. Preferably, thebaking losses of the CED material should amount to not more than 15%,preferably not more than 13%.

POWDER EXAMPLE

Preparing an Epoxy-polyester Powder Coating Material

Into a primary mixer there are introduced 30 parts of polyester resinUralac P 5980 (polyester resin from DSM, having an acid number of70-85), 24 parts of epoxy resin Epikote 1055 (epoxy resin from Shell,having an epoxy equivalent weight of 850), 6 parts of a leveling agentmasterbatch Epikote 3003 FCA-10, 0.2 part of a polypropylene waxLancowax PP1362, 0.4 part of diphenoxy-2-propanol (degassing agent), 30parts of titanium dioxide and 10 parts of calcium carbonate and thesecomponents are premixed. In an extruder, this premix is dispersed atoperating temperatures between 100 and 130° C. and, following dischargefrom the extruder die, is cooled as rapidly as possible over quenchingrolls. Milling is carried out in classifier mills. A classified particlesize adjustment has been found to be particularly favorable.

Line 24 The radiator is then electrostatically coated externally withpowder coating material.

Parameters: gun voltage from 50 to 90 kilovolts, gun/radiator distancefrom 15 to 45 cm.

What is claimed is:
 1. A process for the multilayer coating ofsubstrates with electrodeposition and powder coating materials,comprising a) applying at least one coat of an electrodeposition coatingmaterial to a substrate, b) drying partially or wholly the at least onecoat of the electrodeposition coating material at a temperature of ≦100°C., c) applying at least one coat of powder coating material to the atleast one coat of an electrodeposition coating material, and d) jointlybaking the at least one coat of an electrodeposition coating materialand the at least one coat of powder coating material, wherein drying iscarried out until the difference in weight between the driedelectrodeposition coating material and the baked electrodepositioncoating is less than 20%.
 2. The process of claim 1, wherein the dryingof the at least one coat of electrodeposition coating material takesplace by blowing with air at temperatures of ≦40° C.
 3. The process ofclaim 1, wherein drying lasts ≦60 minutes.
 4. The process of claim 1wherein the joint baking of the electrodeposition coating material andpowder coating material takes place at temperatures from 150 to 220° C.5. The process of claim 4, wherein the joint baking takes place for aduration of from 10 to 40 minutes.
 6. The process of claim 4, whereinthe joint baking takes place for a duration of from 15 to 30 minutes. 7.The process of claim 1, wherein the powder coating material is appliedby electrostatic adhesion.
 8. The process of claim 1, wherein theelectrodeposition coating material crosslinks at a temperature less than170° C.
 9. The process of claim 1, wherein the powder coating materialhas a crosslinking temperature of from 10 to 60° C. above thecrosslinking temperature of the electrodeposition coating material. 10.The process of claim 1, wherein the powder coating material comprisesone or more degassing agent in a concentration of up to 2% by weight.11. The process of claim 10, wherein the powder coating materialcomprises degassing agents comprising compounds of the formula

in which R is analkanol having 1-6 carbon atoms and R₁ and R₂ arebenzoyl- or phenyl groups, and where R₁ and R₂ can be identical ordifferent.
 12. A layered material comprising at least two coats on asubstrate, which is prepared according to the process of claim
 1. 13.The layered material of claim 1, having an electrodeposition coatingmaterial with a thickness of from 5 to 35 μm.
 14. The layered materialof claim 13, having an electrodeposition coating material with athickness of from 10 to 25 μm.
 15. The layered material of claim 1,having a powder coating material with a thickness of from 30 to 200 μm.16. The layered material of claim 15, having a powder coating materialwith a thickness of from 50 to 120 μm.
 17. The process of claim 1wherein the substrate comprises one or more metals.
 18. The process ofclaim 17, wherein the metal substrate is selected from the groupconsisting of iron, zinc, and mixtures thereof.
 19. The process of claim1, wherein the optional drying takes place at temperatures of ≦40° C.20. The process of claim 1, wherein drying lasts ≦30 minutes.
 21. Theprocess of claim 1 wherein the joint baking of the electrodepositioncoating material and powder coating material takes place at temperaturesfrom 160 to 200° C.
 22. The process of claim 1, wherein the powdercoating material is applied by electrostatic adhesion selected from thegroup consisting of high voltage and frictional charging.
 23. Theprocess of claim 1, wherein the electrodeposition coating materialcrosslinks at a temperature of from 140° C. to 160° C.
 24. The processof claim 1, wherein the powder coating material has a crosslinkingtemperature of from 10 to 40° C. above the crosslinking temperature ofthe electrodeposition coating material.
 25. The process of claim 1,wherein the powder coating material comprises one or more degassingagents in a concentration of 0.4% by weight.
 26. A process for themultilayer coating of substrates with electrodeposition and powdercoating materials, comprising a) applying at least one coat of anelectrodeposition coating material to a substrate, b) optionally dryingpartially or wholly the at least one coat of the electrodepositioncoating material at a temperature of ≦100° C., c) applying at least onecoat of powder coating material to the at least one coat of anelectrodeposition coating material, and d) jointly baking the at leastone coat of an electrodeposition coating material and the at least onecoat of powder coating material, wherein the powder coating materialcomprises a film-forming material comprising: A) from 35 to 92.2% byweight of a carboxyl-containing polyesters haivng an acid number of10-150 mg of KOH/g, B) from 0.8 to 20.1% by weight of low molecular masscuring agents containing epoxide groups, C) from 3.7 to 49.3% by weightof epoxy-functional polyacrylate resins having an epoxide equivalentweight of 350 to 2000, and D) from 0.5 to 13.6% by weight of lowmolecular mass compounds selected from the group consisting ofdicarboxylic acids, polycarboxylic acids, dianhydrides, polyanhydrides,and mixtures thereof.
 27. The process of claim 26, wherein drying iscarried out until the difference in weight between the driedelectrodeposition coating material and the baked electrodepositioncoating is less than 20%.
 28. The process of claim 27, wherein drying iscarried out until the difference in weight between the driedelectrodeposition coating material and the baked electrodepositioncoating is less than 13%.